Methods of improving the magnetic quality of anisotropic permanent magnets containing iron, nickel, cobalt, and aluminum



Patented June 8, 1948 METHQDS- QF IMLROVINGF THE MAGNETIC- QUALITY OFANISOTROPIC PERMANENT. MAGNETS CONTAINING. IRON NICKEL, GOBAIIIT', ANDALUMINUM William, 0. Ellis, Maplewood, and Lawrence Ferguson, SouthOrange, N. .L, assignors to Bell Telephone Laboratories, Incorporated,New York, N. Y., acorporationd NewYork No Drawing. Application-September9, 1943,

ser al No. 501,674;

2 Claims. (01. 148-10) This invention relates to the production ofpermanent magnets, more especially of the types which consistprincipally of iron, nickel, cobalt, andalu-mi-num, andspeoifically thetypes which are cooled in a magnetic field to give them increased valuesof magnetic energy.

In a patent to G. V. Jonas, 2,295,082, dated September 8, 1942, aredescribed improved permanent magnets having a tBI Dmx valuejlyin between3,000,000- and 5,0009% 38 com ared to previously produced permanentmagnets inwhich the (3K)max value generally didnot exceedabout 2,000,000or at the most 3,000,000. Magnets prepared according to the Jonas methodcomprise variousproportions of between 16 per cent to 30 per centcobalt, 12 per cent to 20 per cent nickel, 6 per cent to 11 per centaluminum, and the balance chiefly iron. The compositions may, however,include copper or titanium in amounts up to 7 per cent copper or percent titanium or both, of these elements. The general subject,-matteroj. the Jonas patent has been known for some time, having becomeavailable by the publication of British Patent 522,731, accepted June26, 1940. During this time considerable efiorts have been expended inproducing magnets of various sizes and dimensions with high values of(BH)msX. In many cases where size or weight is an importantconsideration in a piece of apparatus or equipment, a correspondinglyhigh value of (BI'Dmax is likewise of importance. These attempts toproduce products such as one would expect to produce did not achieve auniform degree of success. It was found, for example, that there wereunexpected variations in quality of magnets and that the expected highvalues of (BH)max were not always achieved.

Thus, for example, magnets having a nominal composition of 51 per centiron, 13 /2 per cent nickel, 24 per cent cobalt, 8 per cent aluminum and3 per cent copper were found to vary in (BI'Dmax value from .25 10 to4.6x 10 For these magnets, virgin commercial aluminum, electrolyticcopper, electrolytic nickel, and cobalt in the form of rondelles refinedfrom African ore were employed. The product was unexpectedly variableand much of it unsatisfactory in quality.

As part of a manufacturing process the magnets were cast, thenmaintained for from one to two hours at a temperature of from 1290 to1320 C. and then cooled in a magnetic field of about 1,000 oersteds at arate such that they reached a temperature of about 600 C. in ninemonths. In order to insure this rate of cooling, the specimens werecovered with an' asbestos shield or other suitable protective materialwnne'appud to the poles of an electromagnet. After'being taken from theelectromagnet at around500ito 600 C-.- the specimens were'cooledin airat room temperature and then treated by aging for'eight hours at 595 C;It is not necessary, howeveiflto cool the magnets to room-temperaturebfore aging at 595 C.

The non-uniformity of the product led to the supposition that the heattreatment at 1290" to 1320 C. had been unsatisfactorily or ineifectivelyP m d an i -memes le ei s a i s we eretreaiecl 9 or-mare rees wi h; heresu t that n alm st; a in a s themsenet c r n ri e' were rea y inn dvd-l In n tances, imp o em n w s. noted f om ea h reaction' of t e heaieil ent un i immanen qual y o th specimen be ame x ellen It! the earl sages Q hi deve eem e the re son; t r th n qvementli ual t attribu d wmore fective he t r atment we er i 'wa found that this assumption waserroneous and the discovery upon which the present invention is basedwas made.

It was discovered that the improvement which was achieved byreheat-treating specimens resulted from the elimination of carbon whichwas contained as an impurity. It has been established by controlledexperiments that harmful amounts of carbon in the alloys can be removedby suitable heat treatment. For instance, in the composition, 51 percent Fe, 13 per cent Ni, 24 per cent Co, 8 per cent Al, and 3 per centCu, 0.1 per cent carbon reduces the (BH) max value from a normal of 5.0)(10 (for nearly carbon free material) to approximately 1.0 10Maintaining the specimens for about six hours at 1315 0., however,reduces the carbon content of the alloy to below .02 per cent and bringsthe (BI'Dmax value back to normal. The mechanism by which carbon isremoved from the alloy during heat treating is believed to be byoxidation to carbon monoxide and the effusion of carbon monoxide alonggrain boundaries. The oxygen supply is believed to be obtained from theoxides which form in the alloy during melting and casting. This heattreating operation may be carried out at temperatures between about1,200 C. and about 1,350 C.

- So far as is known at the present time the difference between 0.012per cent carbon and the smallest amount which has been achieved, namely0.005 per cent carbon, does not appear to be substantial as thedifference in (BH) max at 0.012

per cent carbon and the smallest carbon content which could be achievedis no greater than about per cent. As the carbon content increases, themagnetic quality decreases rapidly so that at about 0.084 per centcarbon the value of (BI-l) max is less than one-fourth of that achievedwhen the carbon content is kept low. As the carbon content increases, afurther rapid reduction in the magnetic quality occurs.

Some typical experimental data are given in the following table.

Fullness Factor Ha Oersteds Per C ent Carbon 13., (BHM Gausses SpecimenNo. G-O X PPS"? cm- HO cut-paws In the above table:

Bm, measured in gausses, is maximum magnetiza- V tiona Hc, measured inoersteds, is coercive force.

Br, measured in gausses, is residual magnetization.

(BH) max, measured in millions of gaussesoerstads, is maximum energyproduct.

The fullness factor is the ratio Dm, B,H,

What is claimed is:

1. A method of improving the magnetic quality of a magnet formed of analloy consisting essentially of about 51 /2 per cent iron, 13 /2 percent nickel, 24 per cent cobalt, 8 per cent aluminum and 3 per centcopper and having initially a carbon content greater than .012 per cent,said magnet having been produced by subjecting a body of said alloy to acycle comprising heating at a temperature between about 1290 C. andabout 1320 C. for a period of time between about 4 1 hour and about 2hours, cooling said body to about 600 C. in a magnetic field having astrength of about 1000 oersteds at a rate such that it reached600 C. inabout 9 minutes and aging at about 595 C. for about 8 hours, which.method comprises again subjecting said magnet body to said cycle byreheating said magnet and again maintaining it at a temperature betweenabout 1290 OJand about 1320 C. for between about 1 hour and about 2hours, cooling said body to about 600 C. in a magnetic field having afield strength of about 1000 oersteds at a rate such that it reaches 600C. in about 9 minutes and aging said body at about 595 C. for about 8hours. J x

, 2. The method defined in claim 1 wherein the .magnet is subjected tomore than one repetition of the heat treating cycle set forth in saidclaim;

WILLIAM C. ELLIS. LAWRENCE FERGUSON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

